Electrochemical cell component or other material having oxidation preventive coating

ABSTRACT

A body of carbonaceous or other material for use in corrosive environments such as oxidizing media or gaseous or liquid corrosive agents at elevated temperatures, in particular in molten salts such as cryolite, is coated with a protective surface coating which improves the resistance of the body to oxidation or corrosion and which may also enhance the bodies electrical conductivity and/or its electrochemical activity. The protective coating is applied in one or more layers from a colloidal slurry containing reactant or non-reactant substances, or a mixture of reactant and non-reactant substances, in particular mixtures containing silicon carbide and molybdenum silicide or silicon carbide and silicon nitride, which when the body is heated to a sufficient elevated temperature reaction sinter as a result of micropyretic reaction and/or sinter without reaction to form the protective coating.

FIELD OF THE INVENTION

The invention relates to bodies of materials such as, for example,carbonaceous materials, for use in corrosive environments such asoxidising media or gaseous or liquid corrosive agents at elevatedtemperatures, coated with a protective surface coating which improvesthe resistance of the bodies to oxidation or corrosion and which mayalso enhance the electrical conductivity and/or electrochemical activityof the body.

BACKGROUND OF THE INVENTION

Carbonaceous materials are important engineering materials used indiverse applications such as aircraft bodies, electrodes, heatingelements, structural materials, rocket nozzles, metallurgical crucibles,pump shafts, furnace fixtures, sintering trays, inductions furnacesusceptors, continuous casting dies, ingot molds, extrusion canistersand dies, heat exchangers, anodes, high temperature insulation (porousgraphite), gas diffusers, aerospace structural materials, bearings,substrates in electronics industry, brazing and joining fixtures,diamond wheel molds, nozzles, glass molds etc. Although carbonaceousmaterials have properties which make them useful for the applicationsmentioned above, the resistance to oxidation is one property which haslimited the use of these materials. Much effort is therefore underway toimprove the resistance to oxidation of such materials.

Traditional methods of preventing oxidation of carbonaceous materialshave involved the deposition of adherent and highly continuous layers ofmaterials such as silicon carbide or metals such as aluminum. Thedeposit of such materials has normally been carried out by techniquessuch as vapor deposition (both PVD and CVD) or by electrochemicalmethods. Vapor deposition is an extremely slow and costly process andadditionally may not be carried out for large parts such as electrodes.It is also known to plasma spray alumina/aluminium onto the sides ofcarbon anodes used as anodes for aluminium electrowinning, but thiscoating method is expensive. Other techniques such as electrochemicalmethods are limited in the type of materials that may be applied ascoatings and size limitations again may be present. Sol-gel techniquesare known for the application of coatings. However, it is well knownthat these techniques are not adequate for oxidation protection, becausethey produce extremely thin films, usually of the order of 1 micrometerthick, that are most often porous and have a tendency to delaminateespecially under conditions of thermal expansion mismatch with thesubstrate.

Therefore, there is a need for developing a cost effective versatilemethod for coating carbonaceous materials with an adherent coating thatwill effectively prevent oxidation and the loss of the carbonaceoussubstrate because of rapid or slow burning.

SUMMARY OF THE INVENTION

According to the invention, a protective coating on a body ofcarbonaceous or other material which improves the resistance of the bodyto oxidation, and which may also enhance the bodies electricalconductivity and/or its electrochemical activity is applied from acolloidal slurry containing particulate reactant or non-reactantsubstances, or a mixture of particulate reactant and non-reactantsubstances, which when the body is heated to a sufficient elevatedtemperature form the protective coating by reaction sintering and/or bysintering without reaction.

The coatings of the invention are "thick" coatings, of the order of tensof micrometers thick, and contain refractory particulate materials whichadjust to the thermal expansion mismatch and, in most embodiments, aftersintering or oxidation during use, are able to provide a continuousthick silica layer for oxidation prevention.

The invention is particularly advantageous when the body is made ofcarbonaceous material, for instance petroleum coke, metallurgical coke,anthracite, graphite, amorphous carbon, fulerene such as fulerene C₆₀ orC₇₀ or of a related family, low density carbon or mixtures thereof. Thecoatings are particularly adherent on carbon substrates because the highsurface activity bonds the particles to the carbon.

It is advantageous for bodies of low-density carbon to be protected bythe coating of the invention, for example if the component is exposed tooxidising gas released in operation of an electrolytic cell, or alsowhen the substrate is part of a cell bottom. Low density carbon embracesvarious types of relatively inexpensive forms of carbon which arerelatively porous and very conductive, but hitherto could not be usedsuccessfully in the environment of aluminium production cells on accountof the fact that they were subject to excessive corrosion or oxidation.Now it is possible by coating these low density carbons according to theinvention, to make use of them in these cells instead of the moreexpensive high density anthracite and graphite, taking advantage oftheir excellent conductivity and low cost.

The invention also concerns coated bodies with substrates of a metal,alloy, intermetallic compound or refractory material, to which theprotective coating is applied.

Two types of coatings have been developed and are described in thisapplication. One will be called the micropyretic type and the other thenon-micropyretic type. Micropyretic coatings contain combustiblematerials which provide heat during combustion and also add desiredconstituents to the coating after combustion of the coating. Thenon-micropyretic type does not contain any combustible. Mixtures ofmicropyretic and non-micropyretic coatings are also possible. Bothcoatings involve the application of a colloidal slurry which is appliedto the substrate by painting, spraying, dipping or pouring onto thesubstrate. When several layers of such coatings are applied, it ispossible that some may contain micropyretic constituents and some maynot.

Thus, the applied colloidal slurry may contain micropyretic particulatereactant substances which undergo a sustained micropyretic reaction toproduce for example refractory borides, silicides, nitrides, carbides,phosphides, oxides, aluminides, metal alloys, intermetallics, andmixtures thereof, of titanium, zirconium, hafnium, vanadium, silicon,niobium, tantalum, nickel, molybdenum and iron, the micropyreticreactant substances being finely divided particulates including elementsmaking up the refractory material produced.

Such micropyretic reactant substances may for instance compriseparticles, fibers or foils of Ni, Al, Ti, B, Si, Nb, C, Cr₂ O₃, Zr, Ta,Ti_(O) ₂, B₂ O₃, Fe, Mo or combinations thereof.

It is essential to use colloids and mixtures of colloids for applicationof the coatings. Three types of colloidal processing are possible. Thefirst involves the gelation of certain polysaccharide solutions. This,however, is relatively unimportant to this invention. The other twowhich involve colloids and metal organic compounds are relevant to thisinvention. These two involve the mixing of materials in a very finescale. Colloids are defined as comprising a dispersed phase with atleast one dimension between 0.5 nm (nanometer) and about 10 micrometersin a dispersion medium which in our case is a liquid. The magnitude ofthis dimension distinguishes colloids from bulk systems in the followingway: (a) an extremely large surface area and (b) a significantpercentage of molecules reside in the surface of colloidal systems. Upto 40% of molecules may reside on the surface. The colloidal systemswhich are important to this invention are both the thermodynamicallystable lyophylic type (which include macromolecular systems such aspolymers) and the kinetically stable lyophobic type (those that containparticles).

Insoluble oxides in aqueous suspension develop surface electric chargesby surface hydroxylation followed by dissociation of surface hydroxylgroups. Typical equations could be:

    M(OH) surface+H.sub.2 O⃡MO.sup.- surface+H.sub.3 O.sup.+

    M(OH) surface+H.sub.2 O⃡M(OH.sub.2).sup.+ surface+OH.sup.-

where M represents a metal or a complex cation.

Such surface charges and the London and Van der Waals forces keep theparticles from agglomerating. An adsorbed layer of material, polymer orsurface active agent, modifies the interaction of particles in severalways. In the mixing process described below we introduce new materialsand other agents into the colloids.

Colloids may form through cation hydrolysis. Many metal ions are subjectto hydrolysis because of high electronic charge or charge density.Initial products of hydrolysis can condense and polymerize to formpolyvalent metal or polynuclear ions, which are themselves colloidal.Charge and pH determine the ligands for central cations and theanion/cation ratio controls the degree of polymerization and stabilityof the suspension. The pH could vary from 0-14. A wide range ofpolynuclear cationic hydrolysis products may exist with charge from 2+to 6+. Polynuclear anionic hydrolysis products could also have a widerange of charges.

The formation of colloids involves a starting material for example areagent grade metal sale which is converted in a chemical process to adispersible oxide which forms the colloidal solution on addition ofdilute acid or water. Removal of water (drying) and or removal of theanions from the colloidal solution produces a gel Like product. In themethod of the invention for oxidation resistant coatings, the colloidthus acts as a binder to the other additives and also densifies theproduct formed. The calcination process in air yields an oxide productafter decomposition of salts whereas carbon, silicon, boron etc. may beadded to the colloid to yield a non oxide ceramic in the coating. Thecolloidal solutions may also be blended.

The colloidal carrier--usually colloidal alumina, colloidal ceria,colloidal silica, colloidal yttria or colloidal monoaluminium phosphateand usually in an aqueous medium--has been found to assist in moderatingthe micropyretic reaction and to considerably improve the properties ofthe coating whether produced by reaction sintering or non-reactivesintering. It is however not necessary for all of the applied layers ofthe slurry to have a colloidal carrier. Excellent results have beenobtained using some slurries with a colloidal carrier and others with anorganic solvent. Combinations of a colloidal carrier in aqueous mediumand an organic solvent have also worked well.

In the case of micropyretic coatings an additional step after the dryingof the applied slurry on the coating will be the firing (combustion) ofthe constituents of the slurry by direct flame, concentrated sunlight,plasma, laser, electron beam or by traditional methods such as passing acurrent through the conductive substrate or placing the coated articleinside a furnace at a predetermined temperature or by heating thecoating by an induction method or by radiant heating.

The applied colloidal slurry contains particulate substances whichsinter above a given temperature, in particular reactant and/ornon-reactant substances like silicon carbide that reaction sinter and/orsinter without reaction above 900° C. The coating may be pre-formedprior to use, in which case the reactant and/or non-reactant substanceshave been reaction sintered and/or sintered without reaction to providean adherent coating on the body prior to use. Alternatively, themicropyretic reaction sintering or the non-reactive sintering may takeplace only when the body coated with the coating components is used athigh temperature.

When use of a silicon-carbide-containing coating is contemplated attemperatures below 900° C. then normally micropyretic coatings arepreferred; when use is contemplated at above 900° C. thennon-micropyretic coatings are also acceptable. This is because thecoatings become effective after they sinter. Above 900° C., sinteringmay occur during exposure to the service conditions at the hightemperature. Below 900° C. the micropyretic reaction and the combustioninitiation process will provide the required heat for the sinteringoperation. Nevertheless, it remains possible to sinter non-micropyreticcoatings above 900° C. and then use them below 900° C.

In-situ repair of coatings during use is also contemplated by both typesof coatings.

The constituents of the slurries are:

(a) A carrier, chosen from colloidal liquids which could be colloidalalumina, colloidal ceria, colloidal yttria, colloidal silica, colloidalzirconia or mono-aluminum phosphate or colloidal cerium acetate ormixtures thereof.

(b) A powder additive containing carbides, silicides, borides, nitrides,oxides, nitrides, carbonitrides, oxynitrides, boric acid and its salts,and combinations of these. When choosing combinations of powderadditives the particle size selection is of importance. It is preferableto choose particle size below 100 microns and, when employingcombinations of powder additives, to choose particle sizes which arevaried such that the packing of particles is optimized. For example whenchoosing a composition containing mostly SiC and some MoSi₂ it ispreferable to choose the particle size of the MoSi₂ much smaller (atleast three times smaller) than the SiC. Generally, the ratio of theparticle sizes will be in the range from 2:1 to 5:1, preferably about3:1, for instance with large particles in the range 15 to 30 micrometersand small particles in the range 5 to 10 micrometers.

(c) Metallic particles such as for example Ni, Pt, Al, Cr orintermetallic particles such as for example NiAl, NiAl₃, CrSi, CrB etc.or combinations thereof, in which case the particle sizes will be variedno achieve optimum packing, as for powder additives.

(d) Micropyretic agents. These agents are particles, fibers or foils ofmaterials such as Ni, Al, Ti, B, Si, Nb, C, Cr₂ O₃, Zr, Ta, TiO₂, B₂ O₃,Fe, Mo or combinations which may react to yield heat as well as yieldingclean and nascent products from the combustion. Typical reactions couldbe for example Cr₂ O₃ +2Al+2B which reacts spontaneously to give CrB₂and Al₂ O₃ with a large release of heat. The adiabatic temperature ofsuch a micropyretic reaction is 6500° K. Tables I, II and III give apartial listing of examples of micropyretic reactions and products andthe amount of heat released in each reaction. ΔH (KJ/mole) is theenthalpy release for the reaction and T_(ad) K is the adiabatictemperature (°K.) which is expected to be reached in such reactions.

                  TABLE I                                                         ______________________________________                                        FORMATION OF REFRACTORY COMPOUNDS                                             REACTION         ΔH(KJ/mole)                                                                        Tad K                                             ______________________________________                                        Ti + 2B = TiB.sub.2                                                                            -293.00    3190                                              Zr + 2B = ZrB.sub.2                                                                            -263.75    3310                                              Nb + 2B = NbB.sub.2                                                                            -207.74    2400                                              Ti + B = TiB     -158.84    3350                                              Hf + 2B = HfB.sub.2                                                                            -310.15    3520                                              Ta + 2B = TaB.sub.2                                                                            -193.53    3370                                              Ti + C = TiC     -232.00    3210                                              ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        FORMATION OF INTERMETALLICS                                                   REACTION        ΔH(KJ/mole)                                                                        Tad K                                              ______________________________________                                        Ti + Ni = TiNi  -66.5      1773                                               Ti + Pd = TiPd  -103.4     1873                                               Ni + Al = NiAl  -118.4     1923                                               Ti + Al = TiAl  -72.8      1734                                               Ti + Fe = TiFe  -40.6      1423                                               ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        FORMATION OF COMPOSITES                                                       REACTION            ΔH(KJ/mole)                                                                         Tad K                                         ______________________________________                                        Fe.sub.2 O.sub.3 + 2Al = Al.sub.2 O.sub.3 + 2Fe                                                   -836.00     3753                                          Cr.sub.2 O.sub.3 + 2Al = Al.sub.2 O.sub.3 + 2Cr                                                   -530.00     2460                                          2Cr.sub.2 O.sub.3 + 6Al + 6C =  6500                                          2Cr.sub.2 C.sub.3 + 3Al.sub.2 O.sub.3                                         0.86Ti + 1.72B + 1.48Al =                                                                         -293.00     1450                                          0.86TiB.sub.2 + 1.48Al                                                        Ti + C + 0.68Ni = TiC + 0.68Ni                                                                    -232.00     1370                                          Zr + 2B + Cu = ZrB.sub.2 + Cu                                                                     -263.75     1100                                          ______________________________________                                    

(e) Metal organic compounds principally metal alkoxides of the generalformula M(OR)_(z) where M is a metal or complex cation made up of two ormore elements, R is an alkyl chain and z is a number usually in therange 1 to 12, or alternatively described as solutions in whichmolecules in which organic groups are bound to a metal atom throughoxygen. Examples are silicon tetraisomyloxide, aluminum butoxide,aluminum isopropoxide, tetraethyl orthosilicates, etc. Formates,acetates and acetylacetonates are also considered in this category.

(f) Pyrolizable chlorosilanes, polycarbosilanes, polysilazanes and otherorganosilicon polymers as binders which pyrolize to useful products foroxidation prevention. Such compounds are expected to participate in themicropyretic reaction in a beneficial but complex manner to increase theyield of useful product with a morphology and size which assists inmaking the coating more adherent and tight.

(g) Buffer solutions to modify the pH of the slurry. These are standardlaboratory grade alkalines or acids.

(h) Binding agents such as methyl cellulose, clays like kaolinite,polyvinyl butyral, fused silica and activators, etc.

Considering the above defined constituent groups (a) to (h), theslurries used in the invention are made up of at least one of theadditives from groups (b), (c) and/or (d) in a colloidal carrier fromgroup (a), optionally together with one or more components from groups(e) to (h). Some materials may be present under more than one heading.For instance silica or alumina in colloidal form can be included in thecarrier, and in powder form as additive. Particulate nickel andaluminium can be present as a micropyretic reactant, but in excess ofthe stoichiometric amount, whereby the excess forms a particulateadditive. It is also possible for the powder additive to be the same asthe reaction product of the micropyretic reaction.

The non-reactant substances may comprise antioxidant or oxidationprevention materials such as boric acid and its salts, and fluorides;bonding enhancing materials such as methyl-cellulose, particulatealumina or urea; metallic, intermetallic, semi-metallic, polymeric,refractory and/or ceramic materials such as borides, carbides, nitrides,silicides, oxides, oxynitrides and mixtures thereof; pyrolizablechlorosilanes, polycarbosilanes, polysilanes and other organometalpolymers which pyrolize to useful products for oxidation prevention orenhancing bonding, or their pyrolized products: thermosetting resins;thermoplastic resins; and mixtures thereof.

Examples of thermosetting resins are epoxides, phenolic resins andpolyimides. Examples of thermoplastic resins are polycarbonates, e.g.Lexan™, polyphenylene sulfides, polyether ether ketones, polysulfones,e.g. Udel™, polyetherimides and polyethersulfones.

The coating advantageously contains at least one silicon-containingcompound, which may be included as a reactant and/or as a non-reactant,advantageously in a substantial amount, usually accounting for 30 wt %or more of the coating, advantageously 50 wt % or more. Siliconcompounds when reacted or sintered form on the body a relativelyimpervious silica skin, providing excellent resistance against oxidationand corrosion. Formation of such a silicous skin can be enhanced byincluding colloidal silica in the carrier.

The applied coating for instance contains an least one carbide, nitride,boride or oxide of silicon or combinations thereof, in combination withat least one silicide of titanium, zirconium, hafnium, vanadium,niobium, tantalum, nickel, molybdenum, chromium and iron, or acombination of at least two carbides, nitrides, borides or oxides ofsilicon. One particularity advantageous combination includes siliconcarbide with molybdenum silicide. Another includes silicon carbide withsilicon nitride. These silicon-based combinations can be used alone orin combination with other silicon or non-silicon non-reactants or withmicropyretic reactants, and particularly with colloidal silica in thecarrier. When such coatings are sintered before use in an oxidisingatmosphere, or when such coatings are used in an oxidsing atmosphere,the coatings are converted to produce a relatively impervious silicaskin.

The invention is useful for protecting the various engineering materialsmade of carbon listed at the outset. A main application of the inventionis however for the protection of components of electrochemical cells formolten salt electrolysis which components in use are exposed to acorrosive atmosphere, or to a molten salt electrolyte, such as cryolite,and/or to a product of electrolyis in the cell. Such components are thuscoated with a protective surface coating which improves the resistanceof the components to oxidation or corrosion and which may also enhancethe electrical conductivity and/or electrochemical activity. Theprotective coating is applied from a colloidal slurry containingparticulate reactant or non-reactant substances, or a mixture ofparticulate reactant and non-reactant substances, which when thecomponent is heated to a sufficient elevated temperature, prior to orupon insertion in the cell, form the protective coating by reactionsintering and/or by sintering without reaction.

Such components may have a carhonaceous substrate, or a substrate of ametal, alloy, intermetallic compound or refractory material, to whichthe protective coating is applied. The component may be a cathode or acathode current feeder, an anode or an anode current feeder, e.g. for aHall-Heroult cell, or a bipolar electrode for new cell designs.

The invention is particularly applicable to components which are exposedto corrosive or oxidising gas released in operation of the cell orpresent in the cell operating conditions, the component comprising asubstrate of carbonaceous material (particularly low-density carbon),refractory material or metal alloy that is subject to attack by thecorrosive or oxidising gas and is protected from corrosion or oxidationby the protective surface coating.

The invention also concerns a method of improving the resistance tooxidation or corrosion of a body of material for use in corrosiveenvironments such as oxidising media or gaseous or liquid agents atelevated temperatures, the body being in particular a component of anelectrochemical cell for molten salt electrolysis which component in useis exposed to a corrosive atmosphere, or to a molten salt electrolyteand/or to a product of electrolyis in the cell. This method comprisesapplying to the body a protective coating from a colloidal slurrycontaining reactant or non-reactant substances, or a mixture of reactantand non-reactant substances, followed by heating the body prior to orduring use to a sufficient temperature to cause the reactant and/ornon-reactant substances to reaction sinter and/or to sinter withoutreaction to form an adherent protective coating.

The method of application of the slurry involves painting (by brush orroller), dipping, spraying, or pouring the liquid onto the substrate andallowing for drying before another layer is added. The coating need notentirely dry before the application of the next layer. However if one ormore layers with micropyretic constituents are present, then it ispreferable to dry completely prior to firing. Layers may be added toalready fired coatings either for repair or for additional build up.Even when micropyretic constituents are absent, it is preferred to heatthe coating with a suitable heat source such as a torch (butane oroxyacetylene) , a laser, a furnace, etc., so as to improve densificationof the coating. Heating takes place preferably in air but could be inother oxidising atmospheres or in inert or reducing atmospheres.

The substrate may be treated by sand blasting or pickled with acids orfluxes such as cryolite or other combinations of fluorides and chloridesprior to the application of the coating. Similarly the substrate may becleaned with an organic solvent such as acetone to remove oily productsand other debris prior to the application of the coating. Thesetreatments will enhance the bonding of the coatings to the substrate.

After coating the substrate applied by dipping, painting, spraying orcombinations of such techniques in single or multi-layer coatings anddrying, a final coat of one or more of the liquids listed in (a)-(e) maybe applied lightly prior to use.

More generally, after fabrication and before use, the body can bepainted, sprayed, dipped or infiltrated with reagents and precursors,gels and/or colloids.

Examples of some non-micropyretic slurries and some micropyreticslurries are given in Table IV and Table V respectively.

                                      TABLE IV                                    __________________________________________________________________________    EXAMPLES OF NON-MICROPYRETIC SLURRIES                                                                                      Powder/                          Composition Powder (w %)/Particle Size       Carrier                          Sample                                                                            -200 mesh                                                                           <10 microns                                                                          -325 mesh                                                                           -325 mesh                                                                           -325 mesh                                                                           Carrier, vol %                                                                          g/ml                             __________________________________________________________________________    1   SiC 97.5%                                                                           Si.sub.3 N.sub.4 2.5%                                                                --    --    --    Coll-Silica 50%                                                                         10/6                                                                Coll-Alumina 50%                           2   SiC 90%                                                                             Si.sub.3 N.sub.4 10%                                                                 --    --    --    Coll-Alumina 100%                                                                       10/6                             3   SiC 90%                                                                             Si.sub.3 N.sub.4 10%                                                                 --    --    --    Coll-Silica 100%                                                                        10/6                             4   SiC 90%                                                                             --     Y.sub.2 O.sub.3 10%                                                                 --    --    Coll-Alumina 100%                                                                       10/6                             5   SiC 90%                                                                             --     Y.sub.2 O.sub.3 10%                                                                 --    --    Coll-Silica 100%                                                                        10/6                             6   SiC 92.5%                                                                           Si.sub.3 N.sub.4 2.5%                                                                Y.sub.2 O.sub.3 5%                                                                  --    --    Coll-Silica 50%                                                                         10/6                                                                Coll-Alumina 50%                           7   SiC 90%                                                                             Si.sub.3 N.sub.4 10%                                                                 --    --    --    Coll-Yttria 100%                                                                        10/5                             8   SiC 90%                                                                             Si.sub.3 N.sub.4 10%                                                                 --    --    --    Coll-Ceria 100%                                                                         10/5                             9   SiC 90%                                                                             Si.sub.3 N.sub.4 5%                                                                  Y.sub.2 O.sub.3 2.5%                                                                Al.sub.2 O.sub.3 2.5%                                                               --    Coll-Silica 100%                                                                        10/5                             10  SiC 85%                                                                             Si.sub.3 N.sub.4 5%                                                                  Y.sub.2 O.sub.3 2.5%                                                                Al.sub.2 O.sub.3 2.5%                                                               TiB.sub.2 5%                                                                        Coll-Silica 100%                                                                        10/5                             11  SiC 85%                                                                             Si.sub.3 N.sub.4 5%                                                                  Y.sub.2 O.sub.3 2.5%                                                                SiO.sub.2 2.5%                                                                      TiB.sub.2 5%                                                                        Coll-Alumina 100%                                                                       10/5                             12  SiC 90%                                                                             MoSi.sub.2 5%                                                                        Y.sub.2 O.sub.3 2.5%                                                                Al.sub.2 O.sub.3 2.5%                                                               --    Coll-Silica 100%                                                                        10/5                             13  SiC 85%                                                                             MoSi.sub.2 5%                                                                        Y.sub.2 O.sub.3 2.5%                                                                Al.sub.2 O.sub.3 2.5%                                                               TiB.sub.2 5%                                                                        Coll-Silica 100%                                                                        10/5                             14  SiC 85%                                                                             MoSi.sub.2 5%                                                                        Y.sub.2 O.sub.3 2.5%                                                                SiO.sub.2 2.5%                                                                      TiB.sub.2 5%                                                                        Coll-Alumina 100%                                                                       10/5                             15  SiC 80%                                                                             MoSi.sub.2 10%                                                                       Y.sub.2 O.sub. 3 10%                                                                --    --    Coll-Silica 100%                                                                        10/5                             16  SiC 70%                                                                             MoSi.sub.2 20%                                                                       Y.sub.2 O.sub.3 2.5%                                                                Al.sub.2 O.sub.3 2.5%                                                               TiB.sub.2 5%                                                                        Coll-Silica 100%                                                                        10/5                             __________________________________________________________________________

                                      TABLE V                                     __________________________________________________________________________    EXAMPLES OF MICROPYRETIC SLURRIES                                                                                             Powder/                       Composition Powder (w %)/Particle Size          Carrier                       Sample                                                                            -200 mesh                                                                           <10 microns                                                                          -325 mesh                                                                           -325 mesh                                                                           -325 mesh                                                                           Carrier, vol %                                                                             g/ml                          __________________________________________________________________________    1   SiC 60%                                                                             Si.sub.3 N.sub.4 10%                                                                 Ti 17%                                                                              B 8%  TiB.sub.2 5%                                                                        Coll-Silica 50%                                                                            10/6                                                             Coll-Alumina 50%                           2   SiC 50%                                                                             Si.sub.3 N.sub.4 8%                                                                  Ti 25%                                                                              B 10% TiB.sub.2 7%                                                                        Coll-Alumina 100%                                                                          10/6                          3   SiC 50%                                                                             Si.sub.3 N.sub.4 7%                                                                  TiO.sub.2 25%                                                                       B.sub.2 O.sub.3 15%                                                                 Al 3% Coll-Silica 100%                                                                           10/6                          4   SiC 50%                                                                             TiB.sub.2 10%                                                                        Y.sub.2 O.sub.3 8%                                                                  Ni 22%                                                                              Al 10%                                                                              Coll-Ceria Acetate 100%                                                                    10/6                          5   SiC 50%                                                                             ZrB.sub.2 5%                                                                         Y.sub.2 O.sub.3 2%                                                                  Ti 20%                                                                              Ni 23%                                                                              Coll-Silica 100%                                                                           10/6                          6   SiC 72.5%                                                                           Si.sub.3 N.sub.4 2.5%                                                                Y.sub.2 O.sub.3 5%                                                                  Ti 15%                                                                              Si 5% Coll-Silica 50%/                                                                           10/6                                                             Coll-Alumina 50%                           7   SiC 80%                                                                             Si.sub.3 N.sub.4 5%                                                                  Cr.sub.2 O.sub.3 10%                                                                Al 3% C 2%  Coll-Yttria 100%                                                                           10/5                          8   SiC 50%                                                                             Si.sub.3 N.sub.4 10%                                                                 Ti 28%                                                                              C 7%  BaO 5%                                                                              Coll-Ceria Acetate 100%                                                                    10/5                          9   SiC 50%                                                                             Si.sub.3 N.sub.4 5%                                                                  Ti 26%                                                                              Al.sub.2 O.sub.3 3%                                                                 Al 16%                                                                              Polycarbosilane 20%                                                                        10/5                                                             Coll-Silica 80%                            10  SiC 40%                                                                             Si.sub.3 N.sub.4 5%                                                                  Y.sub.2 O.sub.3 5%                                                                  Ti 37%                                                                              Si 13%                                                                              Coll-Silica 100%                                                                           10/5                          11  SiC 45%                                                                             Si.sub.3 N.sub.4 7.5%                                                                Ti 30%                                                                              SiO.sub.2 2.5%                                                                      B 15% Coll-Alumina 100%                                                                          10/5                          12  SiC 90%                                                                             Zr 4%  Y.sub.2 O.sub.3 2.5%                                                                Al.sub.2 O.sub.3 2.5%                                                               B 1%  Mono-Al-Phosphate 100%                                                                     10/5                          13  SiC 85%                                                                             MoSi.sub.2 5%                                                                        Y.sub.2 O.sub.3 2.5%                                                                Al.sub.2 O.sub.3 2.5%                                                               TiB.sub.2 5%                                                                        Coll-Silica 100%                                                                           10/5                          14  SiC 85%                                                                             MoSi.sub.2 5%                                                                        Y.sub.2 O.sub.3 2.5%                                                                SiO.sub.2 2.5%                                                                      TiB.sub.2 5%                                                                        Coll-Alumina 100%                                                                          10/5                          15  SiC 80%                                                                             MoSi.sub. 2 5%                                                                       Y.sub.2 O.sub.3 10%                                                                 Ti 8% C 2%  Tetraisomyloxide 20%                                                                       10/5                                                             Coll-Silica 80%                            16  SiC 68%                                                                             MoSi.sub.2 20%                                                                       Cr.sub.2 O.sub.3 2.5%                                                               Al 7% C 2.5%                                                                              Coll-Silica 100%                                                                           10/5                          __________________________________________________________________________

The invention will be further described with reference to the followingexamples.

EXAMPLE 1

A non-micropyretic slurry corresponding to sample 9 of Table IV wasapplied to a graphite block 4 cm×4 cm×6 cm by painting to build up alayer of approximately 500 micrometers. This block along with anuncoated block were placed in a furnace under air at 1000° C. Theuncoated block completely burnt in 24 hours. The coated block showed aweight loss of 5% after an exposure of 27 hours. The coating was notedto remain very stable and adherent. Scanning electron microscopepictures of the coating before and after the exposure show that thecoating self densified to protect the carbonaceous substrate.

EXAMPLE II

The test of Example I was repeated but with a micropyretic slurrycorresponding to sample 10 of Table V applied as the coating. The coatedsample when placed in the furnace was noted to ignite. The weight lossafter a 27 hour test was 8%.

EXAMPLE III

A combination of a micropyretic slurry and a non-micropyretic slurry wasapplied to a graphite substrate in the following manner. A slurrycorresponding to sample 13 of Table IV was applied by painting andallowed to build up to 600 microns by applying several coats. Next amicropyretic slurry was applied. This slurry corresponded to sample 11of Table V except that the SiC powder size was chosen to be 1 micron.This slurry was built up to approximately 500 microns in thickness, thusmaking the total thickness of the coating to be 1.1 mm. The micropyreticslurry-deposited layer was then successfully fired by an oxyacetylenetorch.

EXAMPLE IV

A slurry was made containing 40% by volume of sample 1 of thenon-micropyretic slurries in Table IV and 40% by volume of sample 1 ofthe micropyretic slurries in Table V. To this was added 5 % by volume ofcerium acetate (alkoxide), 5% by volume of polysilazanes, 2% buffersolution of pH 10, 3% methyl cellulose and 5% by volume colloidalzirconia. This coating was applied to an anthracite substrate. Afterdrying, the micropyretic slurry-deposited layer was successfully firedby an oxyacetylene torch.

EXAMPLE V

A slurry was made containing 25 gm of TiB₂ (particle size 10 microns)and 15 ml colloidal alumina, 10 g titanium (particle size 44 microns)and 5 g boron (particle size 44 microns). This was then coated on agraphite block as in Example I and subjected to the same testconditions. After 27 hours the loss in weight was 5%. The coating hadchanged in color from black to yellow after 27 hours but remainedadherent and protective of the graphite.

EXAMPLE VI

Example I was repeated but a small crack was noted to be present at thecorner of the rectangular graphite piece. After a 27 hour test it wasnoted that a substantial part of the graphite had now burnt by thepassage of air through the crack. Another similar test was repeated witha crack, but the sample was removed from the furnace after 1 hour andthe crack was healed by the application of the same coating. After 27hours the sample was noted again to have been protected, thus showingthat cracks may be healed if discovered early.

EXAMPLE VII

The slurry of Example IV was mixed with carbon black in the ratio 6 mlof slurry to 1 g of carbon black. This was applied to a graphite block.Further, after drying, the sample was dipped in a mono-aluminumphosphate (MAP) solution and allowed to dry again. The entire assemblywas then placed in a furnace at 1000° C. under air. After 27 hours thecoating was noted to remain protective of the substrate.

We claim:
 1. A body of carbonaceous material for use in corrosiveenvironments such as oxidising media or gaseous or liquid corrosiveagents at elevated temperatures, coated with a non-glassy protectivesurface coating which improves the resistance of the body to oxidationor corrosion and which may also enhance the body's electricalconductivity and/or its electrochemical activity, the protective coatingbeing applied from a colloidal slurry containing particulate reactant ornon-reactant, said slurry forming said non-glassy protective coating byreaction sintering and/or sintering without reaction when the body isheated to a sufficiently elevated temperature.
 2. A coated bodyaccording to claim 1, in which the carbonaceous material is selectedfrom petroleum coke, metallurgical coke, anthracite, graphite, amorphouscarbon, fulerene, low density carbon or mixtures thereof.
 3. A coatedbody according to claim 1, in which the applied colloidal slurrycontains micropyretic reactant substances which undergo a sustainedmicropyretic reaction.
 4. A coated body according to claim 3, in whichthe micropyretic reactant substances react to produce refractoryborides, silicides, nitrides, carbides phosphides, oxides, aluminides,metal alloys, intermetallics, and mixtures thereof, of titaniumzirconium, hafnium, vanadium, silicon, niobium, tantalum nickel,molybdenum and iron, the micropyretic reactant substances comprisingfinely divided particulates comprising elements making up the refractorymaterial produced.
 5. A coated body according to claim 4, in which themicropyretic reactant substances comprise particles, fibers or foils ofNi, Al, Ti, B, Si, Nb, C, Cr₂ O₃, Zr, Ta, TiO₂, B₂ O₃, Fe, Mo orcombinations thereof.
 6. A coated body according to claim 1, in whichthe applied colloidal slurry contains non-reactant substances whichsinter above a given temperature.
 7. A coated body according to claim 1,in which the reactant and/or non-reactant substances reaction sinterand/or sinter without reaction above 900° C.
 8. A coated body accordingto claim 1, in which the reactant and/or non-reactant substances havebeen reaction sintered and/or sintered without reaction to provide anadherent coating on the body prior to use.
 9. A coated body according toclaim 1, in which the non-reactant substances comprise antioxidant oroxidation prevention materials such as boric acid and its salts, andfluorides; bonding enhancing materials such a methyl-cellulose,particulate alumina or urea; metallic, intermetallic, semi-metallic,polymeric, refractory and/or ceramic materials such as borides,carbides, nitrides, silicides, oxides, oxynitrides and mixtures thereof;pyrolizable chlorosilanes, polycarbosilanes, polysilanes and otherorganometal polymers which pyrolize to useful products for oxidationprevention or enhancing bonding, or their pyrolized products;thermosetting resins; thermoplastic resins; and mixtures thereof.
 10. Acoated body according to claim 9, containing metallic particles of Ni,Pt, Al , Cr or intermetallic particles selected from NiAl, NiAl₃, CrSi,CrB, or combinations thereof.
 11. A coated body according to claim 1, inwhich the non-reactant substances are particulates with a particle sizebelow 100 microns.
 12. A coated body according to claim 11, in whichdifferent non-reactant particulate substances have different particlesizes to optimize packing of the particles, with particle size ratios inthe range from 2:1 to 5:1, preferably about 3:1.
 13. A coated bodyaccording to claim 1, in which the coating contains at least onesilicon-containing compound.
 14. A coated body according to claim 13, inwhich the coating contains at least one carbide, nitride, boride oroxide of silicon or combinations thereof, in combination with at leastone silicide of titanium, zirconium, hafnium, vanadium, niobium,tantalum, nickel, molybdenum, chromium and iron, or a combination of atlease two carbides, nitrides, borides or oxides of silicon.
 15. A coatedbody according to claim 14, in which the coating contains siliconcarbide and molybdenum silicide or silicon carbide and silicon nitride.16. A coated body according to claim 1, in which the colloidal slurrycomprises at least one of colloidal silica, alumina, yttria, ceria,thoria, zirconia, magnesia, lithia and hydroxides, acetates and formatesthereof as well as oxides and hydroxides of other metals, cationicspecies and mixtures thereof.
 17. A coated body according to claim 16,in which the colloidal slurry is derived from colloid precursors andreagents which are solutions of at least one salt such as chlorides,sulfates, nitrates, chlorates, perchlorates or metal organic compoundssuch as alkoxides, formates, acetates of silicon, aluminium, yttrium,cerium, thorium zirconium, magnesium, lithium and other metals andmixtures thereof.
 18. A coated body according to claim 17, in which thecolloid precursor or colloid reagent contains a chelating agent such asacetyl acetone or ethylacetoacetate.
 19. A coated body according toclaim 17, in which the solutions of metal organic compounds, principallymetal alkoxides, are of the general formula M(OR)_(z) where M is a metalor complex cation, R is an alkyl chain and z is a number.
 20. The coatedbody of claim 1, which is a component of an electrochemical cell formolten salt electrolysis which component in use is exposed to acorrosive atmosphere, or to a molten salt electrolyte and/or to aproduct of electrolysis in the cell, the component comprising asubstrate of carbonaceous material coated with a non-glassy protectivesurface coating which improves the resistance of the component tooxidation or corrosion and which may also enhance the electricalconductivity and/or electrochemical activity, the protective coatingbeing applied from a colloidal slurry containing particulate reactant ornonreactant substance, or a mixture of particulate reactant andnon-reactant substances, said slurry forming said non-glassy protectivecoating by reaction sintering and/or sintering without reaction when thecomponent is heated to a sufficiently elevated temperature.
 21. Anelectrochemical cell component according to claim 20, in which thecarbonaceous material is selected from petroleum coke, metallurgicalcoke, anthracite, graphite, amorphous carbon, fulerene, low densitycarbon or mixtures thereof.
 22. An electrochemical cell componentaccording to claim 20, in which the applied colloidal slurry containsmicropyretic reactant substances which undergo a sustained micropyreticreaction.
 23. An electrochemical cell component according to claim 22,in which the micropyretic reactant substances react to producerefractory borides, silicides, nitrides, carbides, phosphides, oxides,aluminides, metal alloys, intermetallics, and mixtures thereof, oftitanium, zirconium, hafnium, vanadium, silicon, niobium, tantalum,nickel, molybdenum and iron, the micropyretic reactant substancescomprising finely divided particulates comprising elements making up therefractory material produced.
 24. An electrochemical cell componentaccording to claim 23, in which the micropyretic reactant substancescomprise particles, fibers or foils of Ni, Al, Ti, B, Si, Nb, C, Cr₂ O₃,Zr, Ta, TiO₂, B₂ O₃, Fe, Mo or combinations thereof.
 25. Anelectrochemical cell component according to claim 20, in which theapplied colloidal slurry contains non-reactant substances which sinterabove a given temperature.
 26. An electrochemical cell componentaccording to claim 20, in which the reactant and/or non-reactantsubstances reaction sinter and/or sinter without reaction above 900° C.27. An electrochemical cell component according to claim 20, in whichthe reactant and/or non-reactant substances have been reaction sinteredand/or sintered without reaction to provide an adherent coating on thecomponent prior to use.
 28. An electrochemical cell component accordingto claim 20, in which the non-reactant substances comprise antioxidantor oxidation prevention materials such as boric acid and its salts, andfluorides; bonding enhancing materials such as methyl-cellulose,particulate alumina or urea; metallic, intermetallic, semi-metallic,polymeric, refractory and/or ceramic materials such as borides,carbides, nitrides, silicides, oxides, oxynitrides and mixtures thereof;pyrolizable chlorosilanes, polycarbosilanes, polysilanes and otherorganometal polymers which pyrolize to useful products for oxidationprevention or enhancing bonding, or their pyrolized products;thermosetting resins; thermoplastic resins; and mixtures thereof.
 29. Anelectrochemical cell component according to claim 28, containingmetallic particles of Ni, Pt, Al, Cr or intermetallic particles selectedfrom NiAl, NiAl₃, CrSi, CrB, or combinations thereof.
 30. Anelectrochemical cell component according to claim 20, in which thenon-reactant substances are particulates with a particle size below 100microns.
 31. An electrochemical cell component according to claim 30, inwhich different non-reactant particulate substances have differentparticle sizes to optimize packing of the particles, with particle sizeratios in the range from 2:1 to 5:1, preferably about 3:1.
 32. Anelectrochemical cell component according to claim 20, in which thecoating contains at least one silicon-containing compound.
 33. Anelectrochemical cell component according to claim 32, in which thecoating contains at least one carbide, nitride, boride or oxide ofsilicon or combinations thereof, in combination with at least onesilicide of titanium, zirconium, hafnium, vanadium, niobium, tantalum,nickel, molybdenum, chromium and iron, or a combination of at least twocarbides, nitrides, borides or oxides of silicon.
 34. An electrochemicalcell component according to claim 33, in which the coating containssilicon carbide and molybdenum silicide or silicon carbide and siliconnitride.
 35. An electrochemical cell component according to claim 20, inwhich the colloidal slurry comprises at least one of colloidal silica,alumina, yttria, ceria, thoria, zirconia, magnesia, lithia andhydroxides, acetates and formates thereof as well as oxides andhydroxides of other metals, cationic species and mixtures thereof. 36.An electrochemical cell component according to claim 35, in which thecolloidal slurry is derived from colloid precursors and reagents whichare solutions of at least one salt such as chlorides, sulfates,nitrates, chlorates, perchlorates or metal organic compounds such asalkoxides, formates, acetates of silicon, aluminium, yttrium, cerium,thorium zirconium, magnesium, lithium and other metals and mixturesthereof.
 37. An electrochemical cell component according to claim 36, inwhich the colloid precursor or reagent contains a chelating agent suchas acetyl acetone or ethylacetoacetate.
 38. An electrochemical cellcomponent according to claim 37, in which the solutions of metal organiccompounds, principally metal alkoxides, are of the general formulaM(OR)_(z) where M is a metal or complex cation, R is an alkyl chain andz is a number.
 39. An electrochemical cell component according to claim20, which is a cathode or a cathode current feeder.
 40. Anelectrochemical cell component according to claim 20, which is an anodeor an anode current feeder.
 41. An electrochemical cell componentaccording to claim 20, which is a bipolar electrode.
 42. Anelectrochemical cell component according to claim 20, wherein thecomponent in operation of the cell is exposed to corrosive or oxidisinggas released in operation of the cell or present in the cell operatingconditions, said component comprising a substrate of carbonaceousmaterial, that is subject to attack by the corrosive or oxidising gasand being protected from corrosion or oxidation by the protectivesurface coating.
 43. An electrochemical cell component according toclaim 42, wherein the component has a substrate of low-density carbonprotected by the refractory material.
 44. A body of carbonaceous orother material for use in corrosive environments such as oxidizing mediaor gaseous or liquid corrosive agents at elevated temperatures, coatedwith a non-glassy coating which improves the resistance of the body tooxidation or corrosion and which may also enhance the body's electricalconductivity and/or its electrochemical activity, in which theprotective coating is applied from a colloidal slurry containingparticulate reactant or non-reactant substances, or a mixture ofparticulate reactant and non-reactant substances, said slurry formingsaid non-glassy protective coating by reaction sintering and/orsintering without reaction when the body is heated to a sufficientlyelevated temperature, wherein the non-reactant substances compriseantioxidant or oxidation prevention materials such as boric acid and itssalts, and fluorides; bonding enhancing materials such asmethylcellulose, particulate alumina or urea; metallic, intermetallic,semi-metallic, polymeric, refractory and/or ceramic materials such asborides, carbides, nitrides, silicides, oxides, oxynitrides and mixturesthereof; pyrolizable chlorosilanes, polycarbosilanes, polysilanes andother organometal polymers which pyrolize to useful products foroxidation prevention or enhancing bonding, or their pyrolized products;thermosetting resins; thermoplastic resins; and mixtures thereof.
 45. Acoated body according to claim 44, containing metallic particles of Ni,Pt, Al, Cr or intermetallic particles selected from NiAl, NiAl₃, CrSi,CrB, or combinations thereof.
 46. A body of carbonaceous or othermaterial for use in corrosive environments such as oxidizing media orgaseous or liquid corrosive agents at elevated temperatures, coated witha non-glassy protective surface coating which improves the resistance ofthe body to oxidation or corrosion and which may also enhance the bodieselectrical conductivity and/or its electrochemical activity, in whichthe protective coating is applied from a colloidal slurry containingparticulate non-reactant substances, or a mixture of particulatereactant and nonreactant substances, in which the non-reactantsubstances are particulates with a particle size below 100 microns, saidslurry forming said non-glassy protective coating by sintering withoutreaction when the body is heated to a sufficiently elevated temperature.47. A coated body according to claim 44, in which different non-reactantparticulate substances have different particle sizes to optimize packingof the particles, with particle size ratios in the range from 2:1 to5:1.